The long-term goal is to increase knowledge of mechanisms that regulate blood coagulation so that new antithrombotic therapies and preventative measures can be developed. Defects in coagulation regulation lead to increased risk of vascular disease, a major cause of death. Studies will focus on a novel coagulation regulator, protein Z-dependent protease inhibitor (ZPI) that rapidly inactivates Factor Xa (FXa) in the presence of protein Z, Ca 2+ and phospholipids (PL). Our studies show that ZPI/protein Z also inhibits FIXa. Low protein Z levels are associated with stroke, suggesting physiologic relevance. We hypothesize that: A) ZPI and protein Z are significant down-regulators of both FXa and FIXa; B) Mechanisms of ZPI inhibition differ from those of most serpins; D) ZPI inhibits FIXa and FXa by different mechanisms; and C) Defining ZPI mechanisms and sites of protein-protein interaction may lead to new antithrombotic therapies. Specific aims are: 1) Assess whether ZPI and protein Z are significant inhibitors of FXa and FIXa. We will determine if inhibition of both FXa and FIXa occurs at plasma levels of ZPI/protein Z, prothrombin and FX. We will assess the importance of ZPI inhibition of FXa vs. FlXa and determine if combined inhibitory effects are additive. We will find if FXa-ZPI or FlXa-ZPI are major complexes formed when FIXa or FXa are added to plasma. 2) Define mechanisms for ZPI/protein Z inhibition of FXa. Most serpins do not require a protein cofactor, PL or Ca 2+, and serpin-protease complexes are usually covalent and irreversible. Inhibition of FXa by ZPI seems to differ in these respects. We will elucidate ZPI's unusual mechanisms. We will find if cleavage of ZPI by FXa occurs and if ternary complexes of ZPI-protein Z-FXa can form. 3) Determine how mechanisms of ZPI inhibition of FIXa and FXa differ. FXa inhibition by ZPI strictly requires protein Z and FXa is partly protected from ZPI by FVa. We will extend findings suggesting that ZPI inhibition of FIXa does not require protein Z and that FVIIla enhances ZPI inhibition of FIXa. We will find the reasons for the differences and discover if ZPI inhibition of FIXa differs from that of FXa with respect to binary/ternary complexes, covalency of complexes, or reversibility. 4) Elucidate structure-function relationships for ZPI and protein Z that lead to FIXa or FXa inhibition. We will define some of the regions crucial for interaction between these proteins by performing functional studies with modified proteins, selected mutants of rZPI and peptides representing candidate sequences for interactions. We will find whether a ZPI polymorphism we identified affects function.